Merge pull request #13031 from dgreen-arm/add-feature-experimental-5.15

Mbed OS 5.15: Warn on unrecognised error

tools/config/__init__.py

@@ -1354,10 +1354,13 @@ class Config(object):
         self.cumulative_overrides['features']\
             .update_target(self.target)
 
-        for feature in self.target.features:
+        # Features that don't appear in ALLOWED_FEATURES should be removed
+        # with a warning so that they don't do anything unexpected.
+        # Iterate over a copy of the set to remove them safely.
+        for feature in list(self.target.features):
             if feature not in ALLOWED_FEATURES:
-                raise ConfigException(
-                    "Feature '%s' is not a supported features" % feature)
+                print("[WARNING] Feature '%s' is not a supported feature" % feature)
+                self.target.features.remove(feature)
 
         return self.target.features